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While manufacturers of fixed-wing aircraft have borne the brunt of attacks by the green lobby on the environmental performance of their products, rotorcraft makers have been fortunate to escape any widespread public criticism. For whatever displaced logic, planes are viewed as nasty polluters that should be grounded, while helicopters remain free to zip across our skies with gay abandon.
This disparity hasn’t gone unnoticed at defence and aerospace giant EADS. It has pre-empted any change in public mood by developing a highly innovative diesel-electric hybrid concept helicopter that has the potential to reduce dramatically fuel consumption and emissions. The project is aimed at proving that cleaner, quieter and indeed safer helicopters are possible.
“In recent times, a lot of the effort from helicopter manufacturers has gone primarily into improving the performance of systems and existing engines,” says Peter Jaenker, team leader at EADS Innovation Works, the company’s research and technology network. “But the situation is undoubtedly changing – environmental issues and increasing fuel costs have made us more sensitive to the need to save resources as much as possible. This is leading to new thinking.”
The hybrid helicopter concept features efficient electric motors driving the rotors, combined with diesel engines, to cut fuel consumption and emissions by up to 50% compared to traditional rotorcraft. The main components are multiple diesel-electric motor-generator units, a pair of high-performance batteries, and a power electronics unit controlling the energy flows for best efficiency. Two lightweight opposed piston, opposed cylinder (OPOC) diesel engines offer a fuel economy improvement of up to 30% compared to today’s helicopter turbine engines.
The OPOC engine power output shafts are fitted with advanced, weight-optimised generators delivering electrical current to the power electronics unit. This unit manages the distribution of the electricity to the motors driving the main rotor and the tail rotor, to and from the batteries as well as to the other systems on the helicopter. Take-offs and landings are possible on electrical power alone, resulting in lower noise levels and improved safety.
The hybrid system architecture allows the main rotor and its electrical drive to be tilted forwards during cruise flight. This enables the helicopter’s fuselage to remain at its optimum alignment with the airstream, minimising aerodynamic drag and so reducing the power demand. Since the tail rotor has no mechanical linkage to the main rotor and its power source, it can be turned off at higher speeds. During these flight phases, stability and control as well as balancing of the rotor torque are provided by the aerodynamic properties of the tail fin and rudder.
Jaenker says the system has been optimised for fuel saving. “Unlike on a traditional helicopter, where you would have the tail rotor connected to the same gear as the main rotor, on the hybrid system there is no need for a mechanical link,” he says. “That means you can control the power fully independently. In cruise condition you can save a significant percentage of power by switching off the tail rotor or reducing its speed.”
And he says the OPOC diesel engines are particularly suited to the application. “They are extremely lightweight and therefore very efficient. Also, the hybrid system architecture means that we can easily adapt the rpm speed of the rotor, tilting the main rotor forward to deliver aerodynamic benefits which helps us to reduce power consumption during cruise,” he says.
EADS says that the four independent energy sources of this kind of propulsion system ensure high levels of safety and efficiency. Several kinds of combustion engines could be integrated into such a hybrid system. There are noise advantages, too, with Jaenker predicting an 8-9dB reduction aided by a low-tip speed design.
A model of the concept helicopter will be exhibited at the Farnborough Airshow this month. But Jaenker admits it might be some time before a hybrid helicopter takes to the skies.
“We still have to progress with some serious engineering activities, along with certification and proving of all the technology concepts,” he says. “But, if you look at the history of the helicopter industry, the main players have been very keen to introduce new technology. The response so far has only been positive.”